Smectite clay is a commercially important mineral. With appropriate processing, smectite clays, both synthetic and from naturally occurring bentonites, find extensive application, e.g. as viscosifiers, binders, film formers, fabric softeners and retention aid additives in paper making. These clays are platey-type materials having a micaceous structure. They are highly colloidal and readily swell in water to form viscous, thixotropic gels which renders these clays useful as viscosity builders in the foregoing and in many additional industries and applications, including, for example, in foundry molding compositions, as suspension aids in agricultural sprays, and for thickening cosmetics and personal care products.
Most smectite clays are manufactured and sold as fine powders. As with most minerals, however, these powders are difficult and expensive to handle. They also are subject to considerable "dusting", i.e. evolution of dust during handling, so that they can, in some cases, cause environmental problems, and even health risks. To minimize these problems, many minerals are sold by the manufacturer as high solids aqueous slurries. Such high solids slurries can be easily stored, shipped, transferred, e.g. pumped and metered, with significantly less capital expenditures and many fewer problems than are associated with powdered minerals, as mentioned above.
In most applications, nevertheless, it is not economical to ship smectite clay slurries because of the large quantity of water present in shippable slurries. Since smectites are indeed good viscosifiers, it has not heretofore been practical to produce and ship high solids slurries by the usual methods. In general, only about 8% -10% solids slurries of good quality swelling smectite can be produced in water. Indeed, at solids contents greater than about 8%, the viscosities of the slurries can become so high that they cannot readily be pumped by conventional equipment and gelling upon standing becomes a problem. At higher solids it becomes virtually impossible to form a uniform paste without special equipment.
Thus, there is a need for slurries containing substantially greater than 8% by weight of smectite clay, which have viscosities low enough to allow pumping. In addition, the more such clay which can be incorporated into the slurry the more economical it is to ship the clay since the total weight of the slurry will include less water weight.
Van Fisk, U.S. Pat. No. 4,359,339 proposes the addition of a water soluble aluminum salt, such as aluminum sulfate, to permit the making of pumpable aqueous slurries of bentonite of up to 26% by weight of clay, primarily for use in making foundry sand compositions used in the making of molds. While apparently suitable for the purpose disclosed, aluminum salts have not been found to be satisfactory for making clay slurries of higher solids content, which are frequently desired for many purposes, especially when shipping is involved. Van Fisk also points out that certain other ions, such as calcium ion have been known to depress the gelling properties of bentonites in aqueous slurries, but that the properties are not readily reversible upon later dilution, thus limiting the value of such use. While aluminum ion appears to be less subject to this objection, to a considerable extent it suffers from the same problem. All multivalent cations tend to strongly bind the clay platelets together, which prevents dispersion and inhibits performance in applications where good dispersion is needed (viscosifiers, fabric softeners, retention aids).
In copending application Ser. No. 631,919, filed Dec. 21, 1990, for which I am co-inventor, and the disclosure of which is hereby incorporated by reference, it is disclosed that a smectite clay, such as a bentonite clay, may be provided as an elevated solids aqueous slurry e.g. up to 50% solids, by slurrying the smectite clay in an aqueous medium wherein the aqueous medium has an effective concentration of a salt having a monovalent cation. When such a salt is present in the clay slurry, the clay does not swell appreciably, is essentially inactive, and the slurry can be shipped, pumped and stored without creating a gelling problem. When this slurry is diluted with fresh water at the point of application to make it of the desired solids content for use, e.g. even to less than 1% solids, the smectite clay exhibits all of the desired properties, such as fabric softening, retention, etc. in most applications.
In a preferable procedure pursuant to the Ser. No. 631,919 application, a time-stable, low viscosity, high solids aqueous bentonite slurry is prepared by the steps of first preparing an aqueous solution of a monovalent salt; and thereupon dispersing in the solution at least 8% by weight of a powdered bentonite, by adding the powder to the solution with concurrent low shear mixing. The bentonite is preferably a sodium bentonite, which is added to the solution as from 8 to 50% by weight. The monovalent salt may comprise from about 1 to 35% by weight of the solution, and preferably comprises from 3 to 15%. The monovalent salt is preferably sodium chloride. The salt and clay can also be added to the aqueous phase at the same time. This gives a significant improvement over adding salt to a dispersed clay slurry but is not as effective as dissolving the salt first, and then adding the clay.
In the aforesaid Ser. No. 631,919 application it is also shown that potassium salts can be very effective in producing high solids bentonite slurries of desirably low viscosity. However, such salts are shown in our earlier disclosure to have an unacceptable negative effect on application properties of the said slurries. For example, in the Ser. No. 631,919 application, high solids slurries based upon different monovalent cations were diluted to 5% solids with salt-free water, sheared and viscosity measured. These slurries were then tested for their ability to improve water release from paper pulp slurries used in paper making. This technology is described in U.S. Pat. No. 4,305,781. In this test the shorter the "drainage" time (measured in seconds), the better. The optimum slurry would give low viscosity, low liquid separation and good flowability at high solids. Upon dilution for use in commercial applications, however, the optimum slurry would give high viscosity at 5% solids for rheological application and low drainage rate numbers for paper making applications. The data found in experiments of this type showed that potassium ion had a relatively detrimental effect when measured by such "application properties", thus apparently limiting its usefulness for such purposes.
Dispersant salts are routinely used to reduce the viscosity of various mineral slurries. However, when tetrasodium pyrophosphate, a common dispersant salt for minerals is used in applications such as the aforementioned, only moderately high solids slurries can be produced. Accordingly, in the 631,919 application it was concluded that the highest solids slurries which retained good application properties, were made with sodium chloride.
In accordance with the foregoing, it may be regarded as an object of the present invention, to provide a method for preparing a time-stable low viscosity, high solids aqueous smectite slurry, especially a bentonite slurry, which slurry when diluted for use in various applications such as paper making and the like, provides excellent application properties for such uses.
It is a further object of the invention to provide a method as aforementioned, wherein the bentonite in the resultant slurry, upon dilution with fresh water at the point of use, can be easily dispersed to generate the large surface area needed for maximum performance in many applications.
It is a still further object of the present invention to provide an improved smectite clay slurry of increased solids content, which remains pumpable and shippable without prohibitive gelling, even at the elevated solids content.